Multi-chip modules are assemblies of two or more integrated circuits, capacitors, resistors, or other such devices, each connected to a single substrate. The chips are interconnected to implement a system function of the module. The chips on the module may be simple devices, such as transistors or diodes, or may be medium or large scale integrated circuits.
Although multi-chip modules have many advantages, such as reduced size, reduced signal path length, and convenience, they are difficult to test. The pin spacings among chips are close, often in the order of 0.010 inch or less. Also, the chips may be placed irregularly on the surface of the module. As a result, pin connections from the module to test equipment are difficult because of the large number of contact points, tight geometries, and randomness of contact placement. Testing the substrate connections before the chips are mounted is difficult because of the closeness and randomness of pin mount locations. Also, after chips are assembled onto the module, it is difficult to load test vectors for individual chips, unless extra test pads for each chip have been provided on the module. In short, the "bed of nails" approach to testing components of a printed circuit board is not practical for multi-chip modules.
Although "end to end" functional testing of multi-chip modules is available, this type of testing does not guarantee isolation of faults within a module. Several approaches have been used to isolate faults within a multi-chip module. One approach is to test individual chips by placing additional test components, such as boundary scan test devices, on the module. However, this approach defeats the purpose of multi-chip modules, which is to provide a dense and compact device. Another approach is to test chips by providing them chips with built-in test features. However, not all chips can be expected to have this feature. A need exists for a method of testing each chip on the module and the interconnections among them, as well as the entire module.